1. Field of the Invention
The present invention relates to an oxygen concentration sensing apparatus for sensing the concentration of oxygen in a gas such as an engine exhaust gas.
2. Description of Background Information
An air/fuel ratio control apparatus is utilized to apply feedback control to the air/fuel ratio of a mixture which is supplied to an engine to hold the air/fuel ratio to a target value. This control is executed on the basis of the results of sensing the concentration of oxygen in the engine exhaust gas. Such air/fuel ratio control is employed in order to lower the level of pollutants in the exhaust gas, and also to lower the fuel consumption.
An oxygen concentration sensing apparatus which produces an output signal varying in proportion to the oxygen concentration within a gas under measurement has been developed for use in such an air/fuel ratio control apparatus (Japanese Patent Laid-open No. 58-153155). This oxygen concentration sensing apparatus employs an oxygen concentration sensor having a pair of oxygen ion-conductive solid electrolytic members, each in the form of a flat plate. These solid electrolytic members are disposed in the gas under measurement, with each of the solid electrolytic members having electrodes formed on a front and a rear face thereof, the solid electrolytic members being positioned mutually opposing, with a predetermined restricted region formed between them. One of the solid electrolytic members functions as an oxygen pump element, while the other functions as a sensor cell element for measuring the oxygen concentration ratio. If the oxygen concentration sensor is disposed within the gas which is under measurement, and a current is supplied to pass between the electrodes of the oxygen pump element with the direction of current flow being, for example, such that the electrode of the oxygen pump element which is in the restricted region becomes of negative polarity, then gaseous oxygen within the restricted region adjacent to that electrode of the oxygen pump element will become ionized and will flow through the oxygen pump element to be emitted from the positive electrode side of the oxygen pump element as gaseous oxygen. Variations in the concentration of gaseous oxygen within the restricted region will result in differences in oxygen concentration between the gas in the restricted region and the gas on the exterior of the oxygen concentration sensor, whereby a voltage is developed between the electrodes of the sensor cell element. If the level of the pump current supplied to the oxygen pump element is varied such as to mantain this voltage at a constant value, then assuming operation at constant temperature, the pump current value will vary substantially in a direct proportion to the concentration of oxygen within the gas which is under measurement, and an output proportional to this current is produced which represents an oxygen concentration sensing value.
With such an oxygen concentration sensing apparatus, if an excessively high level of pump current should flow through the oxygen pump element, then oxygen will be produced from the solid electrolytic member of the oxygen pump element itself. This results in a phenomenon referred to as "blackening". If for example the solid electrolytic members are formed of ZrO.sub.2 (zirconium oxide), then when an excessively high level of pump current is supplied to the oxygen pump element, oxygen (O.sub.2) will be produced from the ZrO.sub.2, which will be converted to zirconium (Zr). This blackening phenomenon results in rapid deterioration of tne oxygen pump element, and causes reduced operating effectiveness of the oxygen concentration sensor. To prevent this phenomenon, therefore, it is necessary to hold the level of pump current at a value which is lower than a pump current region within which blackening occurs, this region being referred to in the following as the blackening phenomenon generation region.
FIG. 1 is a diagram showing the relationship between oxygen concentration and the pump current which is supplied to an oxygen pump element, with the voltage V.sub.S developed by the sensor cell element being a parameter. The diagram also indicates the blackening phenomenon generation region. The boundary of the blackening phenomenon generation region is linear, as is the relationship between pump current and oxygen concentration for a specific value of V.sub.S as a parameter, so that it is possible to judge from the value of V.sub.S whether or not the pump current value falls within the blackening phenomenon generation region. Thus, occurrence of the blanking phenomenon can be prevented by reducing the level of pump current whenever the voltage V.sub.S rises above a predetermined voltage, i.e. a voltage level indicating that operation is close to the blackening phenomenon generation region. However with such a method, if the voltage V.sub.S should rise above this predetermined level even momentarily, the pump current will be reduced. Immediately after V.sub.S has thereby been reduced below the predetermined level, the pump current will be increased. In this way the pump current level will fluctuate to such an extent that it will be impossible to reliably sense the oxygen concentration.